Stencil feature in the VFB for care-free rendering against bitmap backgrounds

For users rendering against backplate images provided by the client, it's very much needed to use a background that is completely unaffected by any VFB operators and volumetric materials. In many cases, it is currently not possible to get a 1:1 pixel value match and the impossibility to do so still is an unsolved problem.

Right now, we have a TonemapControl map to remove/mitigate the effects of VFB exposure, tone mapping and LUT operators. However, there are considerable limitations:
• for some LUTs or tone mapping operations, their effect can not be effectively removed
• this introduces the need for further ColorCorrection maps for the background, where any change in the VFB operators might need an update within that CC map, requiring forth-and-back adjustments without a guaranteed 1:1 match
• Global Volume will affect the background without any way to work around it currently
• Using DOF with a camera and background bitmaps on matte objects with a ShadowCatcherMtl will add additional DOF effects on the background (this may be a different request and require a solution on its own since it'd be only partially resolved by a new VFB stencil feature if it's based on alpha only)

A new VFB-based image background with a 'stencil composing' feature would have the following advantages:
• would produce a 1:1 color/levels match without the need for TonemapControl or ColorCorrect maps -> much more straightforward
• would work much more reliably (some LUTs and operations will not be properly removed by the current feature set)
• easier post-production (since right now you need to have a proper un/multiplied alpha workflow which some apps just don't support, for example PS)
• for some cases, you could make sure it's all solved within the VFB, not requiring any additional post-production at all

My idea for the feature would be:
• would work on the alpha channel -> basically a stencil where black alpha values would display original 1:1 bitmap values without being affected by exposure, tone mapping and global volumetric materials
• could be set up in the render dialog instead of the material editor (which would be a more intuitive place for it anyway) or in a new VFB operator providing a map slot that could be fed from the material editor
• simple on/off switch within the VFB (or a matter of enabling/disabling the operator in case it's solved via an operator)
• the original (limited) Tonemap Control map would still be there in case a different workflow is needed

Current limitations that might need additional work for a complete solution:

• For volumetric materials, it might still require additional work where any volumetric material has proper alpha contribution parameters and options
• For proper removal of DOF on the background or matte objects using a ShadowCatcherMtl, additional options within the ShadowCatcherMtl might be required

The current solution is cumbersome and works only partially. Hopefully such a feature can be added to the VFB soon.